1. Field of the Invention
The present invention relates to an infrared signal receiver.
2. Description of the Related Art
FIG. 1 of the accompanying drawings shows in block form a conventional infrared signal receiver. A pulse-position-modulated (PPM) signal which is generated when a carrier having a certain frequency is turned on and off is applied to energize an infrared LED to generate a modulated infrared radiation signal. As shown in FIG. 1, such a modulated infrared radiation signal is received by an infrared radiation sensor 1, and then amplified by an amplifier 2. The amplified signal is a passed through a band-pass filter (BPF) 3 that is tuned to the carrier of the PPM signal, so that unwanted signal components and noise are removed from the signal from the amplifier 2. The BPF 3 supplies an output signal to a detector 4 which detects low and high levels of the PPM signal. An output signal from the detector 4 is shaped in waveform by a waveform shaper 5, which outputs pulses depending on the carrier of the PPM signal from an output terminal 6.
The infrared radiation sensor 1 generally comprises a Pin photodiode.
The BPF 3, the detector 4, and the waveform shaper 5 will be described in detail with reference to FIG. 2 of the accompanying drawings. As shown in FIG. 2, in the BPF 3, a capacitor C1 has a terminal connected to the output terminal of the amplifier 2 and another terminal connected to an input terminal of a buffer 12 and an output terminal of a variable-transconductance amplifier 11 which has a noninverting input terminal and an inverting input terminal. The buffer 12 has an output terminal connected to a noninverting input terminal of a variable-transconductance amplifier 13. The variable-transconductance amplifier 13 has an output terminal connected to an input terminal of a buffer 14 and a terminal of a capacitor C2. The capacitor C2 has another terminal connected to ground. The buffer 14 has an output terminal connected to the inverting input terminal of the variable-transconductance amplifier 11 and a terminal of a resistor R2. The noninverting input terminal of the variable-transconductance amplifier 11 is connected to a positive terminal of a voltage source and a terminal of a resistor R1. The other terminal of the resistor R1 is connected to the other terminal of the resistor R2 and an inverting input terminal of the variable-transconductance amplifier 13. The capacitors C1, C2, the variable-transconductance amplifiers 11, 13, the buffers 12, 14, and the resistors R1, R2 thus connected jointly make up the BPF 3 whose input terminal is provided by the terminal of the capacitor C1 which is connected to the amplifier 2 and whose output terminal is provided by the output terminal of the buffer 14.
The BPF 3 has a tuned frequency f0 expressed by the following equation (1):                     f0        =                              1                          2              ⁢                              xe2x80x83                            ⁢              π              ⁢                              xe2x80x83                            ⁢                                                C1                  xc3x97                  C2                                                              ⁢                      xe2x80x83                    ⁢                                    gm1              xc3x97              gm2                                                          (        1        )            
where C1, C2 represent the respective capacitances of the capacitors C1, C2, and gm1, gm2 represent the respective transconductances of the variable-transconductance amplifiers 11, 13.
The BPF 3 thus amplifies only a signal whose frequency is tuned with the tuned frequency f0 depending on the amount of feedback that is established by the resistors R1, R2.
The output terminal of the BPF 3 is connected to an input terminal of a DC level shifter 15 of the detector 4. The DC level shifter 15 has a pair of NPN transistors Q100, Q101 having respective bases connected to each other. The NPN transistor Q100 has an emitter connected to a constant-current supply 25 and serving as a first output terminal of the DC level shifter 15. The NPN transistor Q101 has an emitter connected to a constant-current supply 26 and serving as a second output terminal of the DC level shifter 15. The first output terminal of the DC level shifter 15 is connected to the base of an NPN transistor Q102 of a differential amplifier. The second output terminal of the DC level shifter 15 is connected to an input terminal of a low-pass filter 16. The low-pass filter 16 comprises a resistor R4 having a terminal which serves as the input terminal of the low-pass filter 16 and another terminal connected to a terminal of a capacitor C3 whose other terminal is grounded. The junction between the resistor R4 and the capacitor C3 serves as an output terminal of the low-pass filter 16. The output terminal of the low-pass filter 16 is coupled to the base of an NPN transistor Q103 of the differential amplifier. The differential amplifier has an output terminal connected to an input terminal 17.1 of a current mirror 17 whose output terminal 17.2 is connected to an output terminal 23.3 of a current mirror 23 and a terminal of a capacitor C4 whose other terminal is grounded. The NPN transistors Q102, Q103 have respective emitters connected to an output terminal 23.2 of the current mirror 23. The current mirror 23 has an input terminal 23.1 connected through a resistor R5 to a voltage supply.
Operation of the detector 4 will be described below with reference to FIGS. 3A though 3D of the accompanying drawings. FIG. 3A shows the waveform of a PPM signal by way of example. As shown in FIG. 3A, the PPM signal comprises on periods where the carrier exists and off periods where only a DC signal exists. The PPM signal is supplied from the output terminal of the BPF 3 to the DC level shifter 15 of the detector 4, and applied to the NPN transistors Q100, Q101. The PPM signal applied to the NPN transistor Q100 is transmitted through the NPN transistor Q100 as an emitter follower to the base of the NPN transistor Q102. The PPM signal applied to the NPN transistor Q101 is transmitted through the NPN transistor Q101 as an emitter follower to the low-pass filter 16 where the carrier of the PPM signal is removed. The PPM signal from the low-pass filter 16 is applied to the base of the NPN transistor Q103. The NPN transistors Q102, Q103 operate as a differential switch. When the base potential of the NPN transistor Q102 is lower than the base potential of the NPN transistor Q103, the NPN transistor Q103 is turned on, allowing a current to flow through the current mirror 17 to the output terminal 17.2 thereof. When the base potential of the NPN transistor Q102 is higher than the base potential of the NPN transistor Q103, the NPN transistor Q103 is turned off, preventing a current from flowing to the output terminal 17.2 of the current mirror 17.
A current I4 which flows from the current mirror 17 when the NPN transistor Q103 is turned on is selected to be larger than a current I3 which flows through the output terminal 23.3 of the current mirror 23. Therefore, when the NPN transistor Q103 is turned on, the capacitor C4 is charged with the difference between the currents I4, I3. When the NPN transistor Q103 is turned off, the capacitor C4 is discharged with the current I3. In each of the on periods of the PPM signal, the capacitor C4 is repeatedly charged with the difference between the currents I4, I3 and discharged with the current I3 according to a sawtooth pattern and becomes high in level. In each of the off periods of the PPM signal, the capacitor C4 becomes low in level as it is only discharged with the current I3. The charging and discharging voltages of the capacitor C4 are expressed as follows:                               Charging          ⁢                      xe2x80x83                    ⁢          voltage                =                                                            I                4                            -                              I                3                                      C4                    xc3x97                      1                          2              ⁢                              f                IN                                                                        (        2        )                                          Discharging          ⁢                      xe2x80x83                    ⁢          voltage                =                                            I              3                        C4                    xc3x97                      1                          2              ⁢                              f                IN                                                                        (        3        )            
where fIN represents the carrier frequency of the PPM signal and C4 represents the capacitance of the capacitor C4.
The charging and discharging signal from the capacitor C4 is applied to the waveform shaper 5. The waveform shaper 5 has a hysteresis comparator 18 whose hysteresis width is selected so as not to respond to the peak value of a sawtooth wave generated by extraneous light noise from an inverter-operated fluorescent lamp or the like. The charging and discharging signal from the capacitor C4 is shaped in waveform by the hysteresis comparator 18, which outputs pulses proportional to the on periods of the PPM signal from the output terminal 6.
FIG. 3B shows the waveform of the signal at the bases of the NPN transistors Q102, Q103. The base of the NPN transistor Q102 is subject to device noise and extraneous light noise during the off periods of the PPM signal or while there is no signal applied to the base of the NPN transistor Q102. The noise passes through the base of the NPN transistor Q103, making the detector output signal low in level thereby to operate the detector 4 in error. To prevent the detector 4 from operating in error, a potential difference is developed between the base-to-emitter voltages VBE of the NPN transistors Q100, Q101 by the difference between currents I1, I2 flowing through the respective NPN transistors Q100, Q101, for thereby applying a DC offset to the bases of the NPN transistors Q102, Q103. The DC offset thus applied serves as a detecting threshold voltage. The output signal of the BPF 3 which is applied to the base of the NPN transistor Q102 has a group delay interval from the end of each of the on periods of the PPM signal, as shown in FIG. 3B. If the detecting threshold voltage is fixed, then the group delay interval is also detected. As a result, as shown in FIG. 3C, output pulses of the waveform shaper 5, which depend on the on and off periods of the PPM signal, are made longer than the on periods of the PPM signal by the group delay interval, resulting in an erroneous operation. To prevent such an erroneous operation, the ratio of the charging and discharging currents of the low-pass filter 16 is set to about 50:1, increasing the detecting threshold voltage depending on the level of the output signal from the BPF 3. In this manner, a high-level portion of the positive value of the output signal from the BPF 3 is detected, whereas the group delay interval is not detected, so that pulses proportional to the PPM signal as shown in FIG. 3D are outputted from the output terminal 6.
According to a conventional transmission format for infrared signals, as shown in FIG. 4A of the accompanying drawings, a continuous-wave PPM signal for 108 ms is transmitted as a one-command carrier. Recently, as shown in FIG. 4B of the accompanying drawings, there has been proposed a transmission format, such as for use in a VTR search mode, according to which a PPM signal is transmitted as a one-command carrier for a period longer than 108 ms. The conventional infrared signal receiver is unable to detect such a PPM signal, and makes the output terminal 6 thereof high in level. Since the conventional infrared signal receiver cannot transmit commands accurately, it causes an apparatus which incorporates the conventional infrared signal receiver to operate in error (see FIGS. 4C and 4D of the accompanying drawings).
Reasons for such an error will be described below. The carrier of the output signal from the BPF 3 is removed by the low-pass filter 16, and the ratio of the charging and discharging currents of the low-pass filter 16 is varied to increase the detecting threshold voltage, as the offset voltage between the base voltages of the NPN transistors Q102, Q103, depending on the output signal from the BPF 3. When the output signal from the BPF 3 is continuously supplied, the detecting threshold voltage increases in level even during the off periods of the PPM signal, as shown in FIG. 4C, until it reaches a value which is the sum of the peak level of the output signal from the BPF 3 and the DC offset. Therefore, the detecting threshold voltage exceeds the output signal from the BPF 3 applied to the base of the NPN transistor Q102 by the DC offset, failing to detect the on periods of the PPM signal, i.e., failing to turn on the NPN transistor Q103.
It is an object of the present invention to provide an infrared signal receiver which prevents a detecting threshold voltage from exceeding an output level of a BPF applied to the base of an NPN transistor even when a PPM signal transmitted as a one-command carrier for a period longer than 108 ms is supplied, and is capable of detecting on periods of the PPM signal and outputting pulses proportional to the on periods of the PPM signal from an output terminal for thereby transmitting infrared commands accurately.
An infrared signal receiver according to the present invention employs an attenuated signal from a band-pass filter as a signal for increasing a detecting threshold voltage of a detector. Specifically, the infrared signal receiver includes an attenuator for preventing the detecting threshold voltage from exceeding an output signal from the band-pass filter which is applied to the detector when the detecting threshold voltage increases.
The infrared signal receiver according to the present invention is capable of detecting a continuous-wave PPM signal that is transmitted as a one-command carrier for a period longer than 108 ms. Since the attenuated signal from the band-pass filter is used as a signal for increasing the detecting threshold voltage, the detecting threshold voltage does not exceed the level of the output signal from the band-pass filter which is applied to the base of an NPN transistor of a differential amplifier in the detector.
The infrared signal receiver can output pulses proportional to on periods of the PPM signal from an output terminal thereof for thereby transmitting accurate infrared commands to an apparatus which incorporates the infrared signal receiver.
The above and other objects, features and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate an example of the present invention.